Project Summary HIV-associated neurocognitive disorders (HAND) and neurocognitive impairment (NCI) remain long-term complications of HIV infection despite effective antiretroviral therapy (ART). Even in early HIV infection and in young adults who comprise 37% of new HIV infections, HAND impacts daily functioning and increases morbidity. This impact of HAND in the young will impact the productivity of the work force worldwide. Currently, our mechanistic insight into HAND pathogenesis in people living with HIV (PLWH) remains limited, and no validated biomarkers exist to diagnose and manage NCI. HAND is believed to result from sustained neuroinflammation. Unfortunately, growing evidence suggests inflammation is modulated by drugs of abuse, including the drug of choice among young adults with HIV: marijuana. Marijuana's immunomodulatory effects are largely anti-inflammatory, including suppression of T cell function and monocyte activation, the latter of which is pivotal to HAND immunopathogenesis. This proposal leverages an existing multi-institutional infrastructure focused on drugs of abuse in adult and adolescent HIV+ patients. Data from our group has found that 1) THC treatment reduces monocyte activation; 2) compared to unimpaired, impaired patients have increased CSF markers of neuronal injury (neurofilament (NFL), and 3) single cell RNA sequencing (scRNA- seq) of cerebrospinal fluid (CSF) from HIV-infected subjects reveals differentiated myeloid cells expressing damage-associated microglia (DAM) genes (APOE and TREM2) and other markers of monocyte activation. This proposal will investigate the role of marijuana in modulating neuroinflammation and HAND and identify a set of candidate biomarkers with potential to perform in the immunomodulatory context of substance abuse. An ideal candidate biomarker is one that can perform in the context of HIV driven inflammation and immunomodulation by drugs of abuse. This proposal will be the first to perform comprehensive, multi-domain immune and transcriptomic profiling to investigate the effects of marijuana on systemic and neuroinflammation, and its impact on cognition in young people living with HIV (YLWH). Within four groups (NCI-/MJ-, NCI+/MJ-, NCI-/MJ+, NCI+/MJ+) we will 1) quantify differences in CNS and peripheral inflammatory biomarkers and markers of neuronal injury; 2) evaluate the potential anti-inflammatory and neuroprotective impact of marijuana use; 3) determine the impact of marijuana on the cerebrospinal fluid (CSF) cellular transcriptome via single cell RNA sequencing (scRNA-seq) to identify inflammatory pathways for future interventions; and 4) using novel PET/MR techniques, quantify microglial activation and neuroinflammation and synaptic density via radiotracers (peripheral benzodiazepine receptor (PBR)111 and UCB-J, respectively). Via these objectives, this proposal will provided needed insight into HAND pathogenesis in the context of drug use. From an increased understanding of its pathogenesis using novel immunological and neuroimaging, results will ultimately help identify modulatory pathways of inflammation. Results from this study will add to our understanding of the interplay between cannabinoids and inflammation both within HIV-infected and uninfected young adults. This proposal has strong potential to lay the groundwork for studies to test novel therapeutics, such as cannabinoid receptor ligands, and clinical interventions to reduce HAND morbidity in young adults with HIV.